1. Field of the Invention
The present invention relates to a material formed from SiC (SiC-formed material), and more particularly to a nitrogen-doped n-type CVD-SiC-formed material of high purity β-type crystals, useful as a material for heaters and dummy wafers for semiconductor devices, as well as other applications such as sceptors, core tubes, and the like.
2. Description of Background Art
The SiC-formed materials, which are obtained by depositing SiC on the surface of a substrate by the CVD (chemical vapor deposition) method, forming a film from the deposited material (filming), and removing the substrate, have denser (poreless) properties, have a higher purity, and exhibit more excellent corrosion resistance and heat resistance than SiC-formed materials produced by a sintering process. Therefore, some former type SiC-formed materials are used as a material for heaters and dummy wafers for the manufacture of semiconductor devices, and other applications such as sceptors, core tubes, and the like. However, when this type of SiC-formed materials are used as heaters for the manufacture of semiconductor devices, for example, resistivity as low as that possessed by SiC produced by the sintering process is required in addition to the above excellent characteristics. When used as a material for dummy wafers, lower light transmittance is required. Thus, SiC-formed materials having satisfactory characteristics commonly usable as a substrate for semiconductor manufacturing cannot be obtained by the conventional SiC filming method using the CVD technique.
A method of applying the CVD-SiC-formed materials to heaters by decreasing the resistivity has been known in the art. International Conference Chemical Vapor Deposition, The Electro Chemical Society, 1975, P749-757 (CVD SiC HEATING ELEMENTS: ALTERATION OF ELECTRICAL RESISTANCE BY DOPING by H. Beutler, S. Oesterle and K. K. Yee) discloses a heater produced by forming a layer of SiC(N) with an SiC(N)/TiN/SiC structure formed to a thickness of 0.44 μm by introducing nitrogen gas at a flow rate of 0.4 l/minute at a temperature of 40° C. into the raw material methyl trichlorosilane (MTS) under conditions of a hydrogen stream of 2.0 l/minute, a vapor deposition temperature of 1400° C. and one atmosphere. However, because it is difficult to arbitrarily control the ratio of the raw material MTS concentration and the nitrogen gas concentration, pores are readily produced inside the film. It is thus difficult to obtain CVD-SiC of dense (or close) properties, exhibiting high gas impermeability.
The following method has been proposed as a method of producing nitrogen-doped CVD-SiC-formed materials by controlling the amount of nitrogen gas to be introduced together with raw material gases. Specifically, the method comprises forming a very thin SiC film on a silicon single crystal substrate by introducing monosilane (SiH4) gas and propane gas as raw material gases, and hydrogen gas as a carrier gas, onto the substrate heated to 900 to 1,200° C., then introducing nitrogen gas as a dopant at a flow rate of 1×10−2 to 1 cc/minute together with the raw material gases at a flow rate of 0.05-0.3 cc/minute (the ratio of the nitrogen gas to the raw material gases is, (1×10−2 to 1 cc/minute)/(0.05-0.3 cc/minute)=0.01/0.1-1/0.6=10 to 167 vol %), to grow an n-type 3C type SiC single crystal film on the substrate. A thin film with a thickness of 0.5-3 μm is grown in one hour (Japanese Patent Publication 43240/1991). The growth rate of the film thickness in this method is very slow (0.5-3 μm/hour). The shortest time for obtaining a film with a thickness of 3 mm, which is a practically required thickness for a heater in a heating apparatus, is 1000 hours (3 mm/3 μm/hour). This poses a problem in terms of productivity.
Polycrystal CVD-SiC containing a Group III element and polycrystal CVD-SiC containing a Group V element have been proposed (Japanese Patent Application Laid-open No. 252307/1991) in an attempt to provide the CVD-SiC with characteristics such as a coefficient of thermal expansion, specific resistance, thermal conductivity, etc. which are special, according to applications. Change in the specific resistance when a Group V element (N) is added was shown, but the results are not necessarily satisfactory when the product is used as a raw material for heaters for semiconductor fabrication devices.
To overcome the above problems in SiC-formed materials which are used particularly as a substrate for semiconductor fabrication devices, the present invention has been achieved by carrying out diversified experiments and investigations on the relationship among the reaction conditions, such as the ratio of concentration of gases introduced into the reactor and the residence time of the gases, various characteristics of SiC-formed materials (films) produced, and the deposition rate.
An object of the present invention is to provide an n-type SiC-formed material exhibiting low resistivity and light go transmittance, in addition to other superior characteristics inherently possessed by CVD-SIC, such as high density (poreless), high purity, etc., and suitably used as a substrate for semiconductor fabricating devices, and a method of manufacturing the SiC-formed material by which the SiC-formed materials are obtained at a high productivity and improved deposition rate.